1. Field of the Invention
The present invention relates to a liquid crystal device having a composite layer, which includes a liquid crystal phase of a liquid crystal material exhibiting a cholesteric characteristic and a resin phase, and a method of producing the same.
2. Description of the Background Art
Various kinds of liquid crystal display devices having a memory effect have already been proposed.
U. S. Pat. No. 3,578,844 has disclosed a liquid crystal device, in which cholesteric liquid crystal material capsuled with a polymer such as gelatin or gum arabi is held between a pair of substrates. This liquid crystal device has a memory effect, and performs a predetermined display when supplied with a voltage, as disclosed therein. The state of the display is stably maintained even after the cease of application of the voltage. This liquid crystal device performs the display by utilizing a difference in quantities of reflected light, which is caused by changing, in accordance with the voltage application, the orientation state of cholesteric liquid crystal material having a selective reflection wavelength in a visible range.
The liquid crystal device, which has the composite layer including the polymer material and the cholesteric liquid crystal material, can perform a high resolution display by simple matrix driving without requiring a memory element such as TFT, MIM. However, it has an insufficient self-holding property or form-keeping property, and therefore is liable to change its display state when a pressure is externally applied thereto. Also, it cannot achieve a sufficient contrast.
A reflective liquid crystal device utilizing selective reflection of the cholesteric liquid crystal material changes the display state in accordance with a planar orientation in which helical axes of liquid crystal molecules are perpendicular to the substrate and a focal conic orientation in which the helical axes are irregularly directed or are substantially parallel to the substrate. If the helical axes of liquid crystal molecules are oriented excessively uniformly in the planar orientation, the displayed appearance is significantly affected by a viewing angle. For example, even if the device is transparent in appearance when viewed perpendicularly to the substrate, it exhibits an opaque appearance as the viewing direction shifts from the direction vertical to the substrate.
Further, in the cholesteric liquid crystal material which selectively reflects the light in the visible range, a color tone can be adjusted by controlling the helical pitch length. However, control of the brightness is difficult, and display in half brightness and multi-tone of brightness is difficult.
Regarding the dependency on the viewing angle, U.S. Pat. No. 5,437,811 has disclosed that the dependency on the viewing angle is improved in a liquid crystal device in which a chiral nematic liquid crystal material containing resin added thereto at a rate of 10% by weight or less with respect to the whole weight and exhibiting a cholesteric characteristic is held between a pair of substrates. In this liquid crystal device, addition of a small amount of resin causes a mutual action between the resin and the liquid crystal material at a region near the resin, and thereby the liquid crystal near the resin exhibits a response to an applied electric field when, compared with the liquid crystal material remote from the resin so that the dependency on the viewing angle is improved.
According to the above method in which a small amount of resin is added to the liquid crystal material, however, a self-holding property cannot sufficiently be achieved because the amount of the added resin is small. Therefore, a state of display is liable to change due to a pressure applied to the substrate surface. Although this problem can be avoided by increasing the amount of the added resin, this causes other problems such as rising of a required drive voltage and lowering of a contrast.
Meanwhile, U.S. Pat. No. 5,473,450 has disclosed a liquid crystal device having a composite film or layer which includes resin partitions formed between pixels or picture elements as well as liquid crystal regions formed between the partitions. This partition structure is formed in such a manner that a solution of liquid crystal material and resin is radiated with ultraviolet rays through a photomask so that a portion radiated with ultraviolet rays is cured to form a resin wall which corresponds to the photomask and forms the above partition structure.
The liquid crystal device having the above partition structure of resin can suppress flow of the liquid crystal material, and therefore has a high self-holding property. Further, the forms of the liquid crystal regions can be uniform and can be positioned accurately. Therefore, drive voltages for the respective liquid crystal regions can be uniform so that it is possible to reduce the drive voltage required for simultaneously operating all the liquid crystal regions. Since each liquid crystal region has a relatively large size defined by the partition, the device can provide a better contrast than a liquid crystal device in which fine liquid crystal regions are arranged in resin portions. Such an advantage can also be achieved that sealing is not required at the periphery of the substrate.
However, these do not overcome the problem that the half tone display and therefore multi-tone display is difficult because the cholesteric liquid crystal material exhibits a bistability.
The resin partitions may be formed by a polymerization phase separating method. However, this method may suffer from such a problem that an uncured raw material of resin remains in the liquid crystal material so that the remaining ingredient lowers the phase transition temperature (clearing point) of the liquid crystal material, and/or shifts the selective reflection wavelength of the liquid crystal material. According to this method, the liquid crystal material is liable to be taken into the resin wall or partition, which may lower the strength, the durability and the adhesivity to the substrate of the resin. Further, the range from which resin material can be selected is restricted to a certain extent.